The present invention relates to a process for bending and quenching a window pane, in which the window pane heated to the bending temperature is bent to the desired shape in a horizontal position using a forming frame and the entire surface of both sides of the bent window pane is then rapidly cooled on the forming frame, in a quenching station which follows the bending station, by blowing cold air using blowing boxes provided with blowing nozzles. The invention also relates to the devices that are needed for carrying out this process.
Processes of this kind are known in various forms. In one such process, disclosed by document DE 2,945,776 C2, the flat window pane to be bent is lifted up using a plate with suckers and is put down on the bending former placed vertically beneath the window pane, the window pane taking the shape of the bending former due to the effect of the kinetic energy of the drop. The bent window pane is brought to the quenching station with this forming frame.
In the process called press bending, like that revealed for example in document EP 0,005,306 B1, the forming frame acts as a press bending former by means of which the window pane is pressed against an upper bending former, the entire surface of which is convex. Next, the bent window pane is brought to the quenching station with this forming frame and is rapidly cooled on it.
The forming frames which are used as support frames for the window panes during the quenching operation must be configured so as not to impede the essential rapid cooling of the edge region of the window pane. This is why they are provided, on their support surfaces, with protuberances, flow channels or recesses so that the blown air directed onto the lower face of the window pane also reaches the edge region of the window pane and brings about the rapid cooling needed for producing the required quench. The forming frames for supporting the window pane during the quench are described, for example, in U.S. Pat. No. 5,118,335 and EP-B-0,186,529.
It has proved to be the case that known processes of the abovementioned type, in which the window panes are bent by means of one and the same forming frame and are quenched while resting on this forming frame, did not always allow satisfactory results to be obtained. Depending on the construction of the forming frame, it may happen, for example, that the window pane quenching which can be achieved in the edge region is too low or that, when sufficient quenching is achieved, the bending operation is the cause of optically undesirable deformations in the edges of the window pane. The optically undesirable deformations mainly arise from the protuberances or recesses in the surfaces for supporting the forming frames which are necessary for the blown air to circulate and because of which the edges of the window pane are subjected to unequal stresses. These unequal stresses are particularly apparent and undesirable when the window panes have to be heated relatively strongly in order to obtain a pronounced curvature and/or when the edges of the window pane are provided with an opaque decorative frame of paint to be cured, so that, because of the increased reflection, these deformations, which are minute in themselves, become particularly apparent. The optically undesirable deformations of this kind are especially noticeable when the window panes are fitted without a frame into the body of the motor vehicle since the edge region of the window pane is then visible right up to the actual edge, at which point the aforementioned deformations are naturally the greatest.
The optically undesirable deformations could of course be avoided, or at the very least greatly reduced, if the surface for supporting the window pane on the forming frame were increased, for example by widening the supporting protuberances and/or by reducing the distance between the supporting protuberances. However, the problem then arises again that the amount of blown air reaching the edge region of the window pane is insufficient, so that the necessary degrees of quenching are not achieved.
The object of the invention is to improve the process of the type described in the preamble so that it allows both manufacture of bent window panes whose edges are free of optically undesirable deformations and allows irreproachable quenching of the bent window panes in their edge region.
According to the invention, this object is achieved in that, while the entire surface of the window pane is being rapidly cooled, the edge regions of the window pane that are covered by the forming frame are subjected to additional blowing of cold air by the suitable supply, to the openings passing through the forming frame, of compressed air at a greater pressure than the ambient pressure which prevails between the blowing boxes.
The suitable supply of additional cold air, at an increased pressure, at the covered edge regions of the window vane, through the forming frame, makes it possible to design the bearing surface of the forming frame in such a way that it no longer produces deformations of the edge of the window pane because of the unequal action of the mechanical deformation forces. In particular, the wide flow channels in the forming frame may be omitted and, instead of them, the forming frame may be provided with holes of relatively small diameter or may be made of a porous material. The compressed air is then supplied to the lower face of the forming frame via a circular delivery channel, simply taking care to ensure that the openings in the forming frame, on the one hand, and the volume flow and pressure of the compressed air, on the other hand, are mutually adapted in such a way that the necessary cooling effect in the regions covered by the forming frame is achieved.